Industrial etching processes are utilized on a large scale in mass production of components. The manufacture of printed circuit boards represents the largest single use of industrial etching processes. A thin layer of copper is applied to a substrate such as epoxy resin or other insulative material. The copper layer is selectively coated in a photographic process with protective etch resist. The copper not covered by the resist is then chemically removed by immersing the circuit board in, or spraying the circuit board with, a suitable etchant which chemically dissolves the exposed copper. Thereafter, the remaining resist pattern is removed by solvent to yield the desired conductive printed circuit pattern.
Heated ferric chloride is a commonly used etchant. Ferric chloride is relatively active for the etching operation and relatively safe for handling by operating personnel.
A solution of ferric chloride in water is sprayed or otherwise brought into contact with the surface of the copper to be etched. The ferric (Fe.sup.+.sup.+.sup.+) ions react with copper and are reduced to ferrous (Fe.sup.+.sup.+) ions whereas the copper (Cu.sup.0) is converted into cuprous (Cu.sup.+.sup.1 ) ions according to the following equation: EQU Fe.sup.+.sup.+.sup.+ + Cu.sup.0 .fwdarw. Fe.sup.+.sup.+ Cu.sup.+
As the cuprous (Cu.sup.+) ion dissolves into solution, it immediately reacts with another ferric ion (Fe.sup.+.sup.+.sup.+) according to the following equation: EQU Fe.sup.+.sup.+.sup.+ + Cu.sup.+ .fwdarw. Fe.sup.+.sup.+ + Cu.sup.+.sup.+
Thus, as more copper is dissolved into the solution, the concentration of ferrous ions increases and the concentration of ferric ions decreases. The remaining ferric ion concentration is a principal factor in the effectiveness of the remaining etched solution. Other variables such as solution density and temperature also alter the effectiveness of the solution. Other industrial etchants, such as CuCl.sub.2, (NH.sub.4).sub.2 S.sub.2 O.sub.7, and ammoniated ClO.sub.2 may be utilized. Similar considerations apply to the effectiveness of these solutions. As the etchant is reduced in the process of etching metal, the effectiveness of the solution decreases.
There are three principal techniques for maintaining a controlled etching process.
The simplest process and the one requiring the least additional equipment is a batch process. In a batch process, the etchant is utilized until the quantity of etched metal in solution renders the solution ineffective for further etching. The time required to etch a given thickness of metal from the workpieces passing through the etchant process varies with the amount of copper in solution. Applicant's previously filed application entitled "Etched Rate Monitor and Etching Control", Ser. No. 575,567, filed May 8, 1975, now abandoned, describes a system for use with a batch etching process whereby the instantaneous etch rate may be determined as a function of the ORP of the solution. The entire specification of this application is hereby incorporated into this application. In this previous application, the absolute value of the ORP (oxidation-reduction potential) is compared to the absolute value of the ORP in a reference solution having the same parameters as the initial concentration in the working etchant. Such a process has particular utility where production rates are highly variable. Since no peripheral equipment is required, there is no investment in equipment not being utilized during a hiatus in production. However, where the production is relatively continuous such a batch process has the disadvantage that as the solution becomes more nearly spent, the etch rate decreases and therefore, the production rate is reduced. For a given production requirement then, such a system must have excess capacity with fresh etchant in order to have adequate capacity with nearly spent etchant. Therefore, there is a need for an etching process that maintains a uniform etch rate.
The most complex process is generally referred to a regeneration. A regeneration process is capable of producing a uniform etch rate. The etched metal, such as copper, is continuously removed (by crystalization for example) and the reduced ion reoxidized to maintain maximum effectiveness for the etchant. Such processes require auxiliary regeneration equipment sufficient to remove etched metal from the solution as fast as it is built up during maximum production rates. Thus, production at anything less than the maximum rate results in inefficient use of the regeneration equipment. Further, such equipment is expensive to purchase initially and must be continuously maintained and calibrated to obtain satisfactorly results.
Rejuvenation is another process with the capability for maintaining a uniform etch rate. In rejuvenation, a portion of the working etchant is periodically removed and replaced by fresh etchant. The process of removing spent etchant results in the removal of some of the copper in solution. At the same time, an oxidation process may be utilized to increase the concentration of oxidized ions to the initial level. Such a process requires monitoring of the amount of etchant removed, the amount of etchant added and the amount and rate of oxidant addition for the reoxidation process. Various parameters are monitored by prior art devices for these purposes. The acidity of the solution is one indication of its state. Similarly, the opacity of the solution may be utilized as a rough indication of the oxidation state. The ORP of the solution may also be utilized to determine when it is necessary to begin the rejuvenation process. However, according to prior art techniques, the ORP must be utilized in comparison to a reference etchant, because of the highly dynamic nature of the ion concentration in industrial strength etchants. In the ferric chloride etchant complex, for example, there are ferric ions, cupric ions and ammonium complexes that produce a continuously varying dynamic solution with the resultant variations in the ORP. Comparison to a reference solution with the same initial parameters, reduces the effect of these variables. However, dependence on a reference solution requires that the fresh etchant added and spent etchant be removed in precise proportions and timed relation. Typically, metering pumps must be utilized so that the etchants removed by pumping and drag out will be precisely balanced in precise stoiciometric amounts by the etchant added. Etchant removal and addition must be carefully coordinated with reoxidation.
A system for employing the ORP in a process for rejuvenation of etchant is described in applicant's co-pending application, Ser. No. 517,665 filed Oct. 24, 1974, now U.S. Pat. No. 3,951,711 issued Apr. 20, 1976, for "System for Maintaining Uniform Copper Etching Efficiency". The entire disclosure of the aforesaid patent is incorporated by this reference. Such a process requires expensive and high maintenance metering pumps. If, for example, it is necessary to replace one of the metering pumps, the whole system must be shut down and preferably the entire solution replaced so that the ORP set point may be initialized on a new solution of fresh etchant. Also frequent recalibrations of the metering pumps and their flow in relation to the oxidant (Cl.sub.2) is required.
Therefore, it is desirable to have an etchant rejuvenation control system for an etchant rejuvenation process that is not dependent upon the absolute value of any solution parameter and which does not require precise metering pumps or the like to coordinate the removal and addition of solution with the addition of oxidant. Such a control system is particularly desirable where it makes it possible for etchant removal, addition and oxidation steps to each proceed at their own optimum rate determined by system operational requirements rather than by a required coordination with the addition, removal and oxidation.